Stripped Down Motor

Date: Wed Mar 19 2003 - 09:11:28 PST

Dear Baldemar --

Let's take the easy part of your question first. It absolutely makes sense
that increasing the number of coils would increase the motor speed. The
additional coils increase strength of the magnet formed by the
current-carrying coil, resulting in a stronger interaction between the coil
and the permanent magnet, which in turn would increase the speed. (The
slightly higher resistance due to the slightly longer length of wire would
probably not be a significant factor in reducing the current through the coil
-- the current also affects the magnetic strength of the coil.)

Your voltage results surprise me, but there are a few things which might
explain them. Normally, increasing the voltage should increase the current,
which increases the magnetic strength of the coil, which increases the speed.
One possibility why this didn't happen for you is that your coil got hot,
significantly increasing its resistance, and limiting the current (the
smaller the wire size, the more likely this is). Another possibility has to
do with your batteries or power supply. D, C, AA and AAA batteries are all
1.5 volts. But D cells are capable of delivering much higher currents when
required to. It's something like having a very small pipe and a very large
pipe, both with the same water pressure. When you open the faucet all the way
on both of them, you get a lot more water out of the large pipe. So it's
possible that your batteries or power supply could not deliver the higher
currents asked of them at the higher voltage, and thus the magnetic strength
of the motor coil did not increase as expected. 6 volts from a power supply
rated at 10 amps maximum will likely be much more effective at running the
motor than 6 volts derived from from four 1.5 volt AA batteries connected in
series. Still another consideration is internal resistance in the batteries.
As the batteries become depleted, they may still read 1.5 volts with no load
if tested with a voltmeter, but when required to furnish current, they are
effectively dead, since much of the 1.5 volt potential drop occurs over the
internal resistance inside the battery itself instead of over the external
load (the motor in this case). Batteries should be tested with a battery
tester (inexpensive ones are available at Radio Shack), not just a voltmeter.

Hope all this helps. Good luck!

Don Rathjen
Exploratorium Teacher Institute

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